# Energy/Velocity of a free electron?

1. Sep 21, 2009

### HMS-776

First of all I want to thank everyone here!!! You have all helped me understand many basic and complex physics principles, to which I am very grateful for.

If an atom is ionized in a high voltage field, the free electrons are attracted toward the positive charge. I have heard that the velocity of the electron is directly related to the strength of the positive charge (which makes sense), and this also tells us the energy of the electron.

If the electron is attracted toward a charge of 100V, what would the energy of the electron be, in eV?

If this electron were to collide with an atom would it's full energy be given to the atom to cause collosional excitation and possibly ionization?

2. Sep 21, 2009

### Bob S

Hello HMS again-
The Geiger Muller tube for detecting ionizing radiation (cosmic rays, etc.) works on the principle of attracting free electrons to the positive center wire. because the geometry is cylindrical, the electric field varies as 1/r. For a 2-cm duameter GM tube, the center wire might be 0.1 or 0.2 mm. The free electron undergoes many inelastic collisions as it approaches the center wire, and its maximum energy is a few eV, but not enough to ionize gas molecules. When it gets close, the electric field is high enough that the electron gains many eV between collisions, and collisions are ionizing, thus creating more electrons. Before reaching the center wire, 100's or 1000's of free electrons are created, and an arc is created. Because the voltage to the center wire is fed through a high resistance (~10 megohm), the voltage on the center wire drops, and the arc is extinguished, and the voltage recovers.
Bob S

3. Sep 22, 2009

### HMS-776

Wow that's awesome. Thanks again!